Stabilized polypropionaldehyde containing triphenylphosphine and dilauryl thiodipropionate as stabilizers



United States Patent STAHEELHZED POLYPRGfiHGNALDEHYDE tCQN- TAHNING TRHPHENYLPHGSPHENE AND DI- LAURYL THIUDHPRUPEONATE AS STABI- LHZIERS Eli Perry, Galveston, Tern, assignor to Monsanto Company, a corporation of Delaware No Drawing. Fiied Oct. 18, 1962, Ser. No. 231,546 1 (Ilairn. Cl. 26t)45.85)

The present invention relates to the stabilization of high polymers of aldehydes. More particularly, it relates to the stabilization ofpolyaldehydes against deterioration at high temperatures with a novel synergistic combination of stabilizers.

It is known that aldehydes such as acetaldehyde, propionaldehyde, acrolein, methacrolein, and substituted methacroleins, and the like can be polymerized to yield 'acetal-type polymers of high molecular weight having molecular chains consisting of a succession of alternate carbon atoms and oxygen atoms as represented by the structural formula L JD wherein R is an unsubstituted or a substituted hydrocarbon radical. The utility of these polymers, however, is seriously limited by reason of their instability. The polymers are readily subject to degradation via several different mechanisms: thermal depolymerization initiated at the ends of the polymer molecules; thermal decomposition at elevated temperatures with reformation of the monomeric aldehyde; oxidative attack by oxygen or free radicals having an oxidative tendency; and acid cleavage often brought about by residual traces of the acid catalyst employed in the preparation of the polymer.

Stabilization of the polyaldehydes can be effected by reacting the polymers to tie up the end groups. For example, the polymers can be esterified by reacting them with carboxylic acid anhydrides, preferably acetic anhydride, in the presence of acid-binding substances. These processes, however, are complicated and expensive and the end product is still unstable by virtue of its fundamental polymer chain structure. Another method of protecting the polymeric materials against attack whether the end groups have been reacted or not is by the addition or incorporation of organic compounds as stabilizers in the polyaldehydes. Recently, certain novel classes of stabilizers have been found which can be incorporated into polyaldehydes to produce compositions which are more stable than the polyaldehydes alone and consequently more useful. Now it has been discovered that particular combinations of these stabilizers impart to the polyaldehydes a very high degree of stability to deterioration at elevated temperatures. This improved stability is substantially greater than the additive effect of the individual compounds comprising the stabilizer combination. Hence, the stabilizer combination is referred to as a synergistic combination.

It is a particular object of the present invention to provide a novel synergistic combination of stabilizers for polyaldehydes. It is another object of the invention to provide novel polyaldehyde compositions which are stabilized against the degradative effects of heat by means of a synergistic combination of compounds.

These and other objects and advantages of the invention which will become apparent from the following description thereof are accomplished by incorporating in or intimately admixing with a polyaldehyde a minor amount of a stabilizer combination comprising a triarylphosphine and a bis-(alkyl)-thiodipropionate. Compositions com- 3,262,9ifi Patented July 26, 1966 prising a polyaldehyde and a minor amount of a stabilizer combination comprising a triarylphosphine and a bis- (alkyl)-thiodipropionate exhibit a high degree of resistance of degradation or deterioration when subjected to elevated temperatures.

Any triarylphosphine in which the aryl groups contain from 6 to 12 carbon atoms can be employed with a bis- (alkyl)-thiodipropionate in the stabilizer combination of the invention. Representative examples of suitable compounds include triphenylphosphine, tritolylphosphine, trixylylphosphine, trinaph thylphosphine, diphenyltolylphosphine, benzyldiphenylphosphine, phenyldiriaphthylphosphine, phenylditolylphosphine, and the like.

The bis-(alkyl)-thiodipropionates which can be employed as the second component in the stabilizer combination of the invention are those wherein the alkyl group contains from 8 to 18- carbon atoms. Representative examples of such suitable compounds include bis-(octyl)- thiodipropionate, bis (nonyl) thiodipropionate, 'bis- (decyl)-thiodipropionate, bis-(dodecyl)-thiodipropionate, bis-(tridecyl)-thiodipropionate, bis (myristyl) thiodipropioniate, bis (pentadecyl) thiodipropionate, bis- (cetyl)-thiodipropionate, bis-(heptadecyl) thiodipropionate, bis-(lauryl)-thiodipropionate, and bis-(stearyl)-thiodipropionate. The preferred stabilizer combination is that comprising .triphenylphosphine and bis-(dodecyl)- thiodipropionate.

The amount of the stabilizer combination of the invention employed can be varied widely depending upon the particular polyaldehyde to be stabilized and the degree of stabilization desired. Generally, amounts from as little as 0.01% by weight to as much as 20 fo 25% by weight of the polymer can be employed. For most practical purposes, amounts from about 0.01% to about 15% are satisfactory with amounts from about 3% to about 6% being preferred. The weight ratios of the two components of the synergistic stabilizer combination likewise can be varied. Generally, the combination of the two stabilizer components at a weight ratio of triarylphosphine to bis- (alkyl)-tl1iodipropionate in the range of 1:100 to :1 are utilized and preferably these components are employed in a weight ratio in the range from 1:10 to 10:1.

The synergistic stabilizer combination can be incorporated in the polymer in any of several ways with the two components being incorporated either separately or together. Generally, incorporation is effected by intimately admixing a solution of the stabilizer combination in a volatile solvent with the finely divided polymer and thereafter evaporating the solvent, preferably under reduced pressure. As an alternative method, both the polymer and the stabilizer combination can "be dissolved in a common solvent and the solvent then evaporated. Or, a mutual solution of the stabilizer combination and the polyaldehyde can be made and the mixture then precipitated from the solution by the addition of water or some other non-solvent for the resin and stabilizer combination. Also, the admixture may be effected by dry blending of the stabilizer combination and the polymer on mill rolls. other method, the stabilizer combination can be added to the polymerization system before or after the polymer has formed but before isolation of the polymer.

The polyald'ehydes which can be thermally stabilized according to the invention with a mixture comprising a triarylphosphine and a bis-(alkyl)-thiodipropionate include all those obtained by polymerization of aldehydes of the formula ACHO wherein R can be an alkyl, a cycloalkyl, an alkenyl, an alkynyl, an aryl, an aralkyl, or an alkaryl radical. The stabilizer combination is particularly useful with polyaldehydes derived from saturated aliphatic aldehydes containing from 2 to 10 carbon atoms and more especially from 2 to 7 carbon atoms. Specific examples of suitable aldehyde monomers include acetal- In still andehyde, propionaldehyde, butryaldehyde, valeraldehyde, heptaldehyde, and the like; acrolein, methacrolein, substituted methacroleins such as tiglic aldehyde, crotonaldehyde, and the like; propa-rgyl aldehyde; benzaldehyde, p-tolualdehyde, p-isopropylbenzaldehyde, cinnamaldehyde, phenylacetaldehyde, and the like. Other polyaldehydes with which the stabilizer combinations are useful include those derived from dialdehyd'es such as glyoxaldehyde, succinaldehyde, malealdehyde, glutaraldehyde, and the like; substituted aldehydes such as chloral, aldol, and the like; and heterocyclic aldehydes such as furfural and tetrahydrofurfural. The stabilizer combinations of the invention are likewise useful with copolymers prepared by copolymerizing the foregoing aldehydes with each other and/or with oxy-aldehydes, ether-aldehydes, amino-aldehydes and halogenated aldehydes or with other polymerizable vinyl monomers, epoxides, ketones, and the like.

The compositions of the invention may also include, if desired, plasticizers, fillers, pigments, anti-oxidants, and other stabilizers such as stabilizers against degradation caused by ultraviolet light.

The following example is presented to illustrate the invention but is not to be construed as limiting it in any manner whatsoever. The property of thermal stability which is used in evaluating the effect of the polyaldehyde stabilizer combinations is defined by the value of the reaction rate constant for the thermal degradation of the polymer at a paticular elevated temperature, namely, 190 C. It is well known that chemical reactions may be classed as first order, second order, third order, etc., depending on the number of molecules which enter into or are formed by the reaction. It is also known that the decomposition or degradation of a material following a first order reaction can be expressed mathematically in the form of the differential equation:

in which t is the elapsed time from the beginning of the decomposition reaction, w is the weight of the material which remains undecomposed at time t, and k is a rate constant for the equation. The thermal degradation of the polyaldehyde polymers of this invention has been found to conform generally to a first order reaction which can be expressed by the above equation. The value of k in this equation, therefore, is utilized to characterize the polymers with respect to thermal stability. The lower the numerical value obtained for this constant, the more stable is the polymer.

Example Polypropionaldehyde was prepared by polymerizing propionaldehyde at a temperature of 76 C. in n-hexane as the reaction medium and in the presence of about 9.8% by weight (based on propionaldehyde) of triethylaluminum as catalyst. A portion of the polymer was then stabilized with a mixture consisting of triphenylphosphine and bis-(dodecyl)-thiodipropionate in a weight ratio of 1:1 and subjected to a stabilization test. The procedure for stabilizing and testing was as follows:

A weighed portion of the polymer was placed in a tared 25 cc. Erlenmeyer flask. Toluene (1 cc.) was added to the flask and the polymer was allowed to swell for about 3 hours. Solutions of triphenylphosphine and bis-(dodecyl)-thiodipropionate in toluene of a concentration such as to yield upon drying 3.0% by weight of each of these compounds were then added and the polymer was allowed to soak therein for about 3 hours. At the end of this time, the solvent was evaporated at 25 C. under vacuum. The flask was then purged for about five minutes with argon at a rate of about 250 cc. per minute. The argon flow was reduced to about 5-10 cc. per minute and the flask was immersed in an oil bath maintained at 190 '-1 C. for five minutes while the argon purge was maintained at the slow rate. At the end of this time, the flask was removed from the bath, cooled under argon and weighed to determine any loss in weight. From the weight loss, the percent residue was determined. The first order rate constant, k, was calculated from the data obtained.

For comparative purposes, a sample of the polypropionaldehyde containing no stabilizer, one containing 6% by weight of triphenylphosphine alone and another containing 6% by weight of bis-(dodecyl)-thiodiproprionate alone were subjected under an argon atmosphere to the elevated temperature of the oil bath and the k values for thermal degradation of these polymer compositions were determined in the same manner. Results of these tests tabulated below demonstrate clearly that the combination of triphenylphosphine and bis-(dodecyl)-thiodipropionate is a combination which has a stabilizing eflect greater than the additive or aggregative effect of these individual stabilizers.

Stabilizer (percent by wt.): k None 0.56 6% triphenylphosphine 0.15 6% bis-(dodecyl)-thiodipropionate 0.32 3% triphenylphosphine +3% bis-(dodecyl)- thiodipropionate 0.12

References Cited by the Examiner UNITED STATES PATENTS 2,519,755 8/1960 Gribbins 260-45.85 2,966,476 12/1960 Kralovec et al 26045.9 2,981,716 4/1961 Street et a1. 26045.7 2,985,617 5/1961 Salyer et al 26045.95 3,001,966 9/1961 Funck et a1 26045.9

LEON I. BERCOVITZ, Primary Examiner.

G. W. RAUCHFUSS, JR., Assistant Examiner. 

